Lens distortion algorithm in Movie Clip Editor
they are just the factors of parabolic and quadratic distortion, as well as a distortion of power 6. lens distortion has to be symmetric/isotrtopic, hence it can be written as function of a pixels distance to the center. this distance changes with an towards the center (=distortion).

The new distorted distance to the center is 
D_r(r) = r+ k1*(r)^2 + k2*(r)^4 + k3*(r)^6 

with r being the distance in pixels. So its rather just mathematical parameters, that approximate real lens distortion, than any parameters of real lenses present in your camera.